rowan
Yellowfang and Cinderpelt's was really good, imo! I also kinda liked Lionheart and Graystripe's back in the first book. (If I had to pick a worst, though… *stares at Tigerclaw and Ravenpaw's apprentice-mentor relationship*)
I also feel like Bluestar was a nice decision for Firestar's mentor – sure, it seems a bit “prodigy to be” in the way it's set up, but she taught him some valuable stuff as his mentor!